This invention relates to hemming machines used to join sheet metal panels into a component such as an automobile body assembly. In particular, the invention relates to a method and apparatus for performing a two-stage hemming operation using a single, moveable anvil and two movable dies.
Hemming is a manufacturing technique that is widely used in the automotive industry for joining two sheet metal panels together to form an external body component such as a door, hood or tailgate assembly. Typically, the panel forming the external skin of the component is joined to an inner reinforcing panel around substantially the entire periphery of the component. In the hemming process, this is achieved by folding the outer, peripheral edges of the outer panel over and around the outer edges of the inner panel.
At the beginning of the hemming process, any edge of the outer panel that is to be hemmed is already bent to form a flange which extends generally perpendicular to the main portion of the panel. This flange is usually produced during the stamping operation that forms the outer panel. For purposes of description, it will be assumed that the outer panel is oriented so that the flanges extend upwardly. The inner panel is then placed on top of the outer panel so that it nests within the upturned flanges of the outer panel.
The folding over or hemming of the outer panel flanges is typically completed in at least two stages. In the first stage, a die or tool strikes the flange to bend it inwardly approximately 35xc2x0 to 55xc2x0 from its original vertical orientation. In the second stage, a different die or tool strikes the inward slanting flange to bend it through the remainder of the approximately 90xc2x0 required to make the flange lay flat against and wrap tightly around the edge of the inner panel.
One conventionally known process for performing a two-stage hemming operation on large automotive body components uses two separate presses each including a stationary lower die, commonly known as an anvil, and a vertically movable upper die. The component is placed on top of a first anvil and a first upper die is urged downwardly by a hydraulic or electrically powered press to contact the component and perform the first stage. The component is then removed from the first anvil and placed on a second anvil, which includes a second upper die and is actuated in a similar manner to perform the second stage. This conventional dual press operation incurs the additional cost of purchasing and maintaining two separate presses. Two separate presses also occupy a substantial amount of floor space in a plant.
It is also known to execute a two-stage hemming process using a single hemming press in which movable, cam-operated tools are used to change the geometry of the dies between strokes. The panels are placed in the anvil, and the press is actuated with the tools in a first position to perform the first stage. The press is opened, the tools are moved to a second position, and the press is actuated a second time to perform the second stage. In such a process, a finished component is ejected from the machine every two times the press is actuated.
As described above, the anvil of a conventional hemming press is stationary and an upper die is urged downwardly by a press to hem the component. An adhesive is typically applied between the panels in the area of the hem to ensure a tight seal between the panels and to prevent corrosion-causing moisture from entering the component. As the component is hemmed, a small amount of excess adhesive squeezes out of the joint and sticks to the upper die which contacts the inner panel.
This excess adhesive must be periodically removed from the die, and this is normally accomplished by manually scraping the die while it is in a raised position. In the conventional hemming press described above, the scraped-off excess adhesive falls downwardly onto the anvil, so that the anvil must then be cleaned before production can resume.
It is desirable in the present invention to provide a method and apparatus for producing hemmed sheet metal components in a faster and more economical manner than is possible with conventional hemming presses. A hemming machine according to the present invention can include a power-actuated press station which moves an anvil along a press axis between a retracted position and an extended position, a die movable between a first stamping position aligned with the press axis and a loading position wherein the first die is offset from the press axis, and a second die movable between the stamping position and an unloading position wherein the second die is offset from the press axis.
The hemming machine provided by the present invention is operated by loading the component to be hemmed into the first die when it is in the loading position, moving the first die and the attached component to the stamping position and extending the press to urge the anvil into contact with an un-hemmed component to perform the first stage of the hemming operation. The component remains in contact with the anvil as it is lowered, and the first die is then returned to the loading position. The second die then moves from the unloading position to the stamping position, and the anvil is extended again to perform a second stage of the hemming operation. The anvil is then returned to the retracted position, the component remaining in contact with the second die, and the second die is moved to the unloading position where the component is removed from the press.
A hemming machine according to the invention is more compact than the prior designs using two separate presses. The machine and method provided herein offers a short cycle time, since the first stage of the hemming operation can be performed while a completed part is removed from the second hemming die, and the second stage of the hemming operation can be performed while a new part is loaded into the first die.
In a preferred embodiment of the invention, the press axis is oriented vertically and extension of the press moves the anvil upwardly toward the first and second dies, which have their component-contacting surfaces oriented downwardly. The first and second dies are linearly movable along horizontal axes. Accordingly, a component to be hemmed is loaded into a centrally-located press station from a first side of the press, both the first and second stage of the hemming operations take place in the press station, and the finished hemmed component is unloaded from the press station on a different side of the press, creating a very efficient work flow.
According to another feature of the invention, means are provided for holding the component in connection with the first die as it moves from the loading position to the stamping position in the press station, and for releasing the component after the first hemming stage is complete, so that the component remains in contact with the anvil as it is retracted downwardly. In this fashion, the component is automatically transferred from the loading position into the press station by the movement of the first die, and remains in the press station as the first die returns to the loading position and the second die moves into the press station in preparation for the second stage hemming stroke.
The hemming machine of the present invention also preferably includes means disposed on the second die for engaging the component when the component is raised into contact with the second die during the second stroke of the press station, and holding the component in connection with the second die as it moves to the unloading position after completion of the second hemming stroke. In this fashion, the finished component is automatically transferred out of the press station to the unloading station by the movement of the second die.